Passive signal intensifier



1960 H. E. D. scovu. ET AL 2,950,442

ASSIVE SIGNAL NTENSIFIER Filed Aug. 30, 1956 t6 zorcanzwtk TIME c PQWER INPUT FIG. 4

H. 0. SCOV/L INVENTORS- H SE/DEL By A )1. m;

A TTORNEV trite States tent PASSIVE SIGNAL INTENSIFIER Henry E. D. Scovil, Morristown, and Harold Seidel,

Plainfield, Ni, assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Aug. '30, 1956, Ser. No. 607,067

4 Claims. (Cl. 339-5) This invention relates to electromagnetic wave transmission systems and more particularly, to solid state pulse amplifying devices for use in such systems.

It is an object of the present invention to introduce nonlinear attenuation characteristics into electromagnetic Wave energy by means of a passive solid state device.

It is a more specific object of the invention to intensify the power level of a pulse of microwave energy by means of a power-saturable transmission medium.

It has been observed that certain microwave transmission media exhibit absorption effects due to the coupling of certain of the elementary particles of the media to the microwave signal under the influence of a magnetic or an electrostatic field. Among these media are included ferromagnetic, paramagnetic and resonant dielectric material. Not only do many of these materials provide a suitable transmission medium for electromagnetic Wave energy, but under particular conditions these materials will absorb large amounts of energy from the wave due to a resonant effect. In a ferromagnetic medium, for example, resonant absorption will take place when the medium is biased by an applied magnetic field to gyromagnetic resonance. That is, if the natural precessional frequency of the unpaired electron spins within the ferromagnetic medium is equal to the frequency of the applied signal, large amounts of energy will be transferred from the signal to the electron spins. The absorption capacity of resonant media is ultimately limited by physical factors and hence at larger power levels power saturation occurs, decreasing the attenuation offered by these materials as the power level is further increased. In the case of ferromagnetic media, for example, the electron spins are decoupled from the applied signal due to a strong interaction with a preferred band of spin waves in the media. This effect is more completely described in applicants copending application Serial No. 607,066, now United States Patent 2,920,292, issued January 5, 1960, filed concurrently herewith. This saturation effect results in an absorption characteristic having a negative slope over a limited range of power levels of the applied signal. A typical negative slope characteristic of a power-saturable resonant medium is shown in Fig. 1. It can be seen from Fig. 1 that at some critical power value P the attenuation (0:) decreased with further increases in power level.

In accordance with the present invention, the negative slope characteristic of power-saturable microwave transmission media is utilized to provide a microwave pulse amplifier. The power-saturable medium is power-biased to a power level just below the critical power level as shown in Fig. l. A pulse superimposed on this bias power triggers the medium onto the negative slope characteristic and thereby decreases the attenuation offered to the signal. A relative amplification takes place if the pulse peak is attenuated substantially less than the bias power. Alternately, this decrease in attenuation can be utilized to trigger the coupling of a high level power source to Patented Aug. 23, 1960 the output and thereby raise the power level at the output during the pulse to many times the power level of the triggering pulse.

It should be remembered that the structures described operate as amplfiers only in a limited sense. That is, the power-saturable medium is unable to see the actual waveform of the applied signal having a period less than the response time of the power-saturable medium, but sees only the average power level of this signal. Since a pulse has the same actual waveform as its power envelope, the structures may correctly be called pulse amplifiers. However, in another sense these structures could be called pulse iutensifiers, or, more more broadly, merely nonlinear attenuators. The term pulse amplifier as hereinafter used will be construed to indicate a device responsive to the power level of a signal and providing a relative power amplification of this signal.

The major advantage of the present invention resides in the extreme ruggedness of the solid state pulse amplifier as compared with prior art devices. This ruggedness allows a much longer operating life in applications where it was heretofore diflicult or even impossible to use microwave components.

These and other objects and features, the nature of the present invention and its various advantages, will appear more fully upon consideration of the accompanying drawings and the following detailed description of these drawings.

In the drawings:

Fig. l is a graphical and qualitative representation of the attenuation versus power input characteristics of a typical power-saturable medium;

Fig. 2 is a perspective view of a first principal embodiment of the invention showing a gyromagnetic pulse amplifier and associated components;

Fig. 3, given for thepurposes of illustration, is a graphical and qualitative representation-of an input and an output pulse from the structure of Fig. 2; and

Fig. 4 is a perspective view of a second principal embodiment of the invention showing an alternative form of a solid state pulse amplifier.

Referring more particularly to Fig. 1, there is shown a graphical and qualitative representation of the attenuation versus applied power characteristic of a typical power-saturable resonant medium. It can be seen that the attenuation offered by such a medium to microwave energy is very high at power levels below a critical value represented by P in Fig. 1. At this critical power level, however, the attenuation begins to decrease, eventually going to relatively low values. It is apparent that every power-saturable medium has a negative slope attenuation characteristic for certain ranges of power level.

Power-saturable microwave transmission media may, for the purposes of the present invention, be construed to mean any microwave transmission media which exhibits a frequency-selective absorption of a microwave signal. These media may therefore be termed powersaturable resonant media. Included in power-saturable resonant microwave transmission media are ferromagnetic, paramagnetic and resonant dielectric materials. Ferromagnetic materials are characterized by a relatively large number of unpaired electron spins capable of interacting with externally applied magnetic fields and, more particularly, are characterized by electron spins in a sufiiciently close coupling relation to one another to produce significant interspin coupling. One of the most important of the resonant ferromagnetic materials is the class of materials comprising iron oxide combined with a small amount of one or more bivalent metals, such as manganese or zinc, in a spinel crystalline structure. These materials are known as ferromagnetic spinels or ferrites. Paramagnetic materials, on the other hand, are characterized by certain unpaired electron spins which interact with a microwave signal but are not significantly coupled to one another. Some of the more important paramagnetic materials are manganesesulfate and gadolinium ethyl sulfate. Resonant dielectric materials are characterized by a resonant absorption of energy under the influence of an electrostatic rather than a magnetic field; Included in this class of materials are most gaseous discharges and, more specifically, ammonia. The term ower-saturable medium, as used in this disclosure, will mean any microwave transmission medium exhibiting resonant absorption of energy at microwave frequencies and will specifically include all ferromagnetic, paramagnetic and resonant dielectric materials. All of these materials exhibit the saturation eifect'illustrated in Fig. l and are capable of utilization in the following described devices.

In Fig. 2 is shown a perspective view of a gyromagnetic pulse amplifier representing a first principal embodiment of the invention. The pulse amplifier shown in Fig. 2 comprises a section of rectangular wave guide 20 having a wider dimension substantially equal to twice the narrower dimension and preferably capable of sup porting only the dominant mode of wave energy therein. Connected to one end of guide 20 is a mixing device 23 having connected thereto a pulse signal source 21 and a microwave power supply 29. Source 21 may, for example, be a transmission circuit for pulse-coded information while power supply 29 may be a microwave oscillator such as a klystron or a magnetron. Mixing device 28 'superimposes the signals from source 21 on a constant power bias supplied by power supply 29. Device 28 may, for example, be a wave guide bridge circuit or coupler into which the outputs of source 21 and supply 29 are fed and out of which a signal is derived representing the sum of these inputs. Connected to the other end of guide 20 is a load 22 to which it is desired to introduce the pulse-coded information in all respects identical to the input signal excepting only that the pulse power content has been substantially increased. Load 22 may, for example, .be a pulse code detector in a pulse code transmission system.

In order to raise the power content of the input pulses from source 21, a gyromagnetic pulse amplifier is included within guide 20 between source 21 and load 22. Centrally located within guide 20 is a slab-shaped element 23 of gyromagnetic material such as, for example, ferrite, having the properties described with respect to Fig. 1. Element 23 extends between the broader walls of guide 20 parallel to and equally spaced from the narrower walls thereof. Element 23 may, however, have any other shape or be placed in any other position within guide 20 so long as it intercepts a substantial portion of the wave energy in guide 20. The ends of element 23 are provided with knife-edge tapers 24 and 25 to prevent undue reflection of wave energy therefrom.

Element 23 is magnetically or electrostatical-ly biased or polarized by a field represented by the vector H in Fig. 2. In the case of a gyromagnetic material, H is a magnetic field which may be supplied by any one of several methods well known to those skilled in the art including an electrical solenoid, an electrically energized magnet, or by permanently magnetizing element 23 it self. The value of the magnetic field H is adjusted so' as to produce gyromagnetic resonance in element 23. The constant power level upon which pulse-coded information is superimposed is adjusted so as to be just below the critical power level P,, as shown in Fig. l. The pulses superimposed on this level have a sufiiciently high power content to raise the overall power level above the critical value'P Under these conditions, the structure in Fig. 2 can be seen to operate as a pulse amplifier or intensifier. This operation can be better understood by considering Fig. 3. e

In Fig. 3 is shown, for the purposes of illustration, a graphical and qualitative representation of a typical input and corresponding output pulse passed through the structure of Fig. 2. Solid curve 26 in Fig. 3 represents an input pulse introduced into the pulse amplifier of Fig. 2 and dashed curve 27 represents the corresponding output pulse derivedfroni the pulse amplifier of Fig. 2. It can be seen that the base power level of the input pulse is just below P while the power level of the pulse peak issubstantially greater than P As shown in Fig. l, the attenuation ofiered to signals having a power level below P is very high while the attenuation offered to signals having a power level above P is relatively low. The output signal, represented by dashed curve 27, is therefore represented as a pulse having a base power level substantially below the input base power level and a peak power level only slightly less than the input peak power level. The power level of the input pulse has been amplifiedto the extent that the attenuation at the base power level exceeds the attenuation at the peak power level.

In Fig. 4 is shown a perspective view of an alternative form of a solid state pulse amplifier comprising two sections 30 and 31'of rectangular wave guide having a common narrower wall 34 between them. Guide 30 and guide 3i are coupled together by a coupling slot 32 in common wall 34. Included in guide 31 in the region of coupling slot 32 is a rectangular element 33 of powersaturable material. Element 33 may, for example, comprise a ferromagnetic, a paramagnetic or a resonant dielectric medium. The pertinent property of element 33 is a negatively sloped attenuation characteristic similar to the characteristic shown in Fig. 1. As a specific example, element 33 may be made of gyromagnetic material magnetically biased to gyromagnetic resonance. Connected to one end of guide 3% is an energy dissipating load 33 which may, for example, be a wedge-shaped block of dielectric material in which carbon particles have been suspended. Connected to the other end of guide 30 is a power source 35, capable ofsupplying microwave energy at a relatively high constant power level. Connected to the corresponding end of guide 31 is a pulse signal source 36 supplying pulse-coded information to guide 31. A load 37 is connected to the other end of guide 31 for utilizing the pulse-coded information at a higher power level than that provided by signal source 36.

The pulse amplifier shown in Fig. 4 operates in the following manner. Element 33 is biased to a resonant state by a magnetic field H or an equivalent electrostatic field in the case where element 33 is a resonant dielectric medium. Under this condition of biasing element 33 presents a high attenuation to electromagnetic wave energy below the critical power P and a low attenuation to energy above the critical power P Coupling slot 32is adjusted to permit suflicient power to be transferred from guide 30 to guide 31 to supply the losses in element 33 and maintain a power level just below the critical value. The balance of the power introduced into guide 36 by power source 35 is dissipated in load 33. The energy coupled to guide 31 is substantially all attenuated by element 33 and hence the output to load 37 is small or negligible.

When a pulse is introduced into guide 31 by signal source 36, the pulse travels down guide 31, encountering element 33. If the peak power level of this pulse is suificient to raise the power level at the leading edge of element 33 above the critical value, this edge of element 33 becomes less dissipative due to the power saturation effect. The lower attenuation in guide 31 allows a greater amount of energy to be transferred from guide 30, causing a further decrease in the attenuation I offered by element 33. The build-up of energy in guide 31 continues until substantially all of the power supplied by power source 35 is being coupled to guide .31. For

a practical pulse amplifier, the power level supplied by power source 35 is much, much greater than that supplied by signal source 36. Load 37 therefore now receives a signal having a power level many times that of the signal introduced by signal source 36.

When the pulse generated by signal source 36 terminates, the power level at the leading edge of element 33 closest to signal source 36 falls below the critical power. Since the power saturation efiect is extremely localized, the state of saturation of element 33 at every point is substantially uncoupled from the saturation state of every other point in element 33. The leading edge of element 33 therefore falls below the critical power level and becomes more dissipative. This in turn allows less energy to be coupled to guide 51 from guide 30, causing a further increase in the attenuation offered by element 33. This decoupling of guides 30 and 31 continues until element 33 is in its entirety in the high loss state. The output to load 37 therefore falls to a small value substantially of the same power level as it was before element 33 was triggered. The transition from one loss state to another is accomplished very rapidly, on the order of milli-microseconds, and hence does not appreciably distort the pulse waveform. It can be seen that the structure of Fig. 4 is a pulse amplifier having an amplification factor substantially equal to the ratio of the power level supplied by power source 35 and the peak power supplied by signal source 36.

The pulse amplifiers described above have been illustrated as utilizing conductively bounded wave guides having rectangular cross-sections. The same principles may, however, be applied to conductively bounded Wave guides of any cross-section, coaxial lines, or dielectric wave guides without a conductive boundary. In these cases, it is merely necessary to substitute the equivalent wave guiding components for the ones shown in the dravw'ngs and to adjust these components to permit operation as described. Furthermore, it has so far been assumed that the p'ower-saturable medium Was entirely dissipative rather than dispersive, and that the entire effect depended on actual power absorption. The effect of poWer-saturable media is actually both dissipative and dispersive and either or both efiects may be used equally well in the embodiments shown in Figs. 2 and 4. In Fig. 2, a large dispersive effect, representing a large phase shift rather than attenuation, would result in reflection rather than absorption of energy in the unsaturated state. Similarly, a large phase constant in the power-saturable medium of Fig. 4 will just as effectively decouple the guides as a large attenuation constant. In the case of a dispersive rather than a dissipative medium, the characteristic shown in Fig. 1 represents the change in phase constant rather than attenuation for a change in power level.

In all cases it is understood that the above-described arrangements are simply illustrative of a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles of those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A pulse intensifier comprising a power-saturable medium magnetically biased to resonance having a first operating region of high attenuation for applied signals below a critical power level and a second operating region of low attenuation relative to the attenuation in said first region for applied signals above said critical power level, high frequency power biasing means for maintaining a given power level in said medium below said critical power level, input means for applying pulses to said medium, said pulses having a power level less than said given power level but suflicient when added to said given power level to raise the power within said medium above said critical power level and into said second region, and output means for receiving power transmitted through said medium.

2. The combination according to claim 1 wherein said medium is located in a first waveguide and wherein said high frequency power biasing means is applied to a second waveguide, and means for directionally coupling said guides to each other.

3. A pulse intensifier comprising a power-saturable medium magnetically biased to resonance having a first operating region of high attenuation for applied signals below a critical power level and a second operating region of low attenuation relative to the attenuation in said first region for applied signals above said critical power level, high frequency power biasing means for maintaining a given substantially constant power level in said medium below said critical power level, input means for applying pulses to said medium, said pulses being superimposed upon said high frequency power bias to raise the power within said medium above said critical power level and into said second region, and output means for receiving power transmitted through said medium.

4. A pulse intensifier comprising a power-saturable medium magnetically biased to resonance having a first operating region of high attenuation for applied signals below a critical power level and a second operating region of low attenuation relative to the attenuation in said first reg on for applied signals above said critical power level, high frequency power biasing means for maintaining a given substantially constant power level in said medium below said critical power level, input means for applying pulses to said medium, said pulses being superimposed upon said high frequency power bias and having a power level less than said given power level but sufiicient when added to said given power level to raise the power within said medium above said critical power level and into said second region, and output means for receiving power transmitted through said medium.

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